1. Field of the Invention
The present invention relates to methods and apparatus for rearranging the sequential time relation between a plurality of discrete data segments occurring within a predetermined period of time. More particularly, the present invention relates to apparatus and methods for receiving a series of blocks of data which occur during a predetermined time frame with those data blocks interchanged or otherwise rearranged in their order of appearance as an output within a time frame of equal length to the input time frame. Although it has utility in purely electronic applications, the present invention is especially useful in optoelectronic and/or photonic environments requiring the handling, processing and/or communicating of photonic data.
Note that while this specification and description contains extensive emphasis on implementation by means of optic and optoelectronic elements and components, the invention is not so limited. Those having normal skill in the art will recognize technologies or combinations of disciplines are available to utilize the invention including all electronic implementations.
2. Description of the Prior Art
Time-slot interchangers accept a serial input signal typically consisting of several equal length blocks of data which are time multiplexed into a larger block of serial data. The time period occupied by the large block is called a frame, while the smaller blocks occupy time-slots within the frame. Multiple frames may follow each other in succession. Corresponding time-slots of successive frames are thought of as belonging to one particular body of data. For example, in the communications environment, a given time-slot in successive frames may contain the digitized version of a particular telephone voice transmission. The output of a time-slot interchanger is a serial signal of the same format as the input, but the data blocks occupying the time-slots are permuted so that a different time-slot within the frame is occupied by the output data. Except for ordering of time-slots, the data is unchanged. Using the communications example, if output time-slots correspond to receivers of compressed voice transmissions, a time-slot interchange represents a mapping, or interconnection, between transmitters and receivers.
A time-slot interchanger is thus the time domain equivalent of a multiple input, multiple output switching network. If there are N time-slots per frame and the interchanger is capable of an arbitrary permutation, there exists the time domain equivalent of an N input, N output crossbar switch. Such a switch is useful wherever independent data items are time multiplexed on a single channel. This could apply within a digital computing system, as well as in a communications network. A time multiplexed channel might connect several subunits of type A to subunits of type B, with a fixed order assigned to the subunits for sending data to, and receiving data from, the channel. An arbitrary permutation performed by a time slot interchanger in the channel would correspond to a random interconnection of type A subunits to type B subunits.
For time-slot interchangers implemented and used with photonic signals, the optical information is carried serially in optical fiber. The basic elements of which the interchangers are built includes fiber delay lines, fiber splitters, and controlled directional couplers. The interchanger system designer should note that controlled directional couplers are expensive, in size, power and cost, compared to the other elements.
For purposes of this description, a serial photonic signal carried by a fiber and organized as a sequence of frames is considered as the input signal. In data communications, various channels may share the same transmission medium by time-division multiplexing, where the different time-slots within each frame are allocated to different channels. In an optical computer, several processes can run simultaneously, and a fiber link between the processors and the memory may carry a time multiplexed data stream, where each time slot within a frame may consist of a word from each of the processors. In either of the above areas, at one or more points between the source and the destination, processing may require an interchange of the time-slots within the frames.
For instance, in a communication network, between every pair of communicating stations, a number of switching nodes may exist which require time-division switching between incoming and outgoing traffic. To obtain massive processing power, an optical computer should not only tap the spatial parallelism but also harness the time domain, thus obtaining the advantages of extremely high speeds and short pulses. Exploiting the time domain would in turn mean that in an optical computer, time-slot interchange may represent a basic functional requirement.
Interchange of digital data from time-slots in the input to time-slots in the output is possible in both a bit-interleaved data format as well as in a word-interleaved data format. In the description contained herein, the word-interleaved data format is assumed throughout although the invention is equally applicable to bit interleave formation. The word-interleaved format seems naturally suited to a switching system whose building block, for example, is a commercially available Ti:LiNbO3 directional coupler. This device is basically electronically controlled, though its switched data is photonic. With word-interleaved data format, a much higher data rate may result than that permitted directly by the electronic switching time of the directional coupler. This is possible because switching needs to take place only between the boundaries of time-slots with the switching time accommodated by providing guard bands of time between neighboring time-slots.
In general, it is assumed there are N time-slots per frame and B bits per time-slot with a data rate of F frames per second. The time-slots are named 0, 1, 2 , . . . , N-1, while the frames are called F.sub.0, F.sub.1, . . . , etc.. For simplicity, the guard band, start bit, etc., are integrated into each time-slot and the frame is shown to consist only of N time-slots. The frame advances so that, as F.sub.k arrives at an input tap, the input receives the data 0, 1, 2 , . . . , N-1 in that sequential order. Hence the data rate is F frames/s, or FN time-slots/s and the time-slot switching rate is Fn Hz. If c is the velocity of light in vacuum and r is the refractive index of the fiber, then the length L of fiber occupied by a time slot is: EQU L=c/FNr
A time-slot interchanger using a number of switches directly proportional to the number of time-slots, N, is presented by R. A. Thompson in "Architectures With Improved Signal-To-Noise Ratio In Photonic Systems With Fiber-Loop Delay Lines", IEEE Journal On Selected Areas In Communications, Vol. 6, No. 7, Aug. 1988. This system receives a data stream which is demultiplexed so as to route segments thereof to various parallel delay lines. Each delay line typically has a different delay magnitude as compared to its neighbors. The outputs of the array of parallel delay circuits are combined through a multiplexer to generate a serial output stream with a rearranged order of time slots relative to the input stream. While functionally acceptable as a time-slot interchanger, such systems are expensive to fabricate and require large bulks of delay lines.
Alternate architectures using much shorter delay lines for time slot interchanging is also known wherein the parallel delay circuits each are configured with a single switch with feedback loop delay or cascade of 2.times.2 switches which are interconnected through a delay. Such variable integer delay systems using demultiplexer, multiplexer and cascade coupled photonic switches are described in the article entitled "An Experimental Photonic Time-Slot Interchange Using Optical Fibers As Reentrant Delay-Line Memories" by Thompson and Giordano, Journal of Lightwave Technology, Vol. Lt-3, No. 1, Jan. 1987, pages 154-161. These systems require even more switches proportional to the number of time-slots/frame and require signal passage through a large number of switch elements.